The invention relates to an axial piston type hydraulic pump/motor which provides an energy translation from or to a static fluid pressure, and more particularly, to an apparatus for supporting piston shoes of the pump/motor in a manner such that they are urged against the surface of a swash plate.
Fluid pressure translating device of the axial piston type is well known in the art to be often usable either pump or motor (e.g. U.S. Pat. Nos. 3,249,061, Ricketts, and 3,522,759, Martin). Such device usually includes a drive shaft which is associated with either a cylinder block or an inclined and inclinable cam plate to create relative rotational movement between the cylinder block and the cam plate. The cylinder block includes a plurality of pistons in cylinders in an annular array about the axis of the cylinder block. The pistons have inner ends disposed for reciprocation within the cylinders and outer ends adapted to bear against and transmit force to or receive force from the inclined cam plate. In such device where the cylinder block is rotatably mounted, the cylinder block includes passages from each cylinder to an end of the block for association with inlet and outlet passages in a port member or port plate. The inlet and outlet passages of the port member communicate successively with the passages from the cylinders upon relative rotation of the cylinder block with respect to the port member. The cylinder block is positively biased toward the port plate by pressure during the operation of device and by mechanical means supplementing the operating pressure, e.g. during starting of the device.
In hydraulic pumps/motors of the kind described, it is well known to provide an arrangement which may be energized by resilient means to urge piston shoes, which are caused to slide along the surface of a tiltable swash plate, against the latter and to prevent their movement away from the swash plate as forces tending to cause such movement are produced during a switching between the high and low pressure in a valve plate which distributes the hydraulic fluid or during a fluctuation of a charging pressure or variation of the higher pressure as the load changes. A variety of such arrangements are provided (e.g. U.S. Pat. No. 3,249,061, in particular, see FIG. 1). If the piston shoes are urged against the tiltable plate with a force of improper magnitude, they may oscillate on the plate, causing a degradation in the performance and the durability thereof. The prior art solution to this problem has been to utilize a force of sufficient magnitude in the support apparatus associated with piston shoes to oppose any force causing an oscillating movement of piston shoes away from the tiltable plate. However, this prevents a proper abutment from being achieved during normal conditions, and also disadvantageously increases the starting torque of the pump/motor.